The arrest of a Belfast man in connection with the attempted murder of IRA informer Martin McGartland sparked a furious protest last night.

The 32-year-old man was arrested earlier yesterday along with a 33-year-old man from Glasgow after a joint operation by the Royal Ulster Constabulary, Northumbria Police and Strathclyde Police.

The two are being questioned at an undisclosed police station in the Northumbria area.

Mr McGartland, aged 29, suffered six gunshot wounds in an attack at his home in Duchess Street in the Tyneside seaside resort of Whitley Bay in June.

Northumbria Police launched a massive manhunt and publicity appeal to find the gunmen, but said they were keeping an open mind on the motive while not discounting possible terrorist involvement.

At the time, Sinn Fein denied any IRA involvement in the attack. Last night, Sinn Fein Assembly member for north Belfast, Gerry Kelly, called for the Irishman's immediate release.

In a statement he said: "This morning's raid is an appalling act which saw one man arrested and his home and business badly damaged.

"Once again the RUC are involved in activities which run counter to the peace process and can only cause difficulties for those attempting to break the present political impasse.

"I am calling for the man's immediate release and an explanation from the NIO Security Minister Adam Ingram. We are treating this incident with the utmost seriousness."

The Royal Ulster Constabulary said it was a matter for Northumbria Police, which is leading the inquiry into Mr McGartland's shooting, to comment.

A Northumbria Police spokeswoman said: "The men were arrested on suspicion of conspiring to murder Mr McGartland under new cross border powers conferred by the Criminal Justice and Criminal Order Act 1994.

Mr McGartland infiltrated the republican movement on behalf of the RUC Special Branch during the 1980s and later published Fifty Dead Men Walking, recounting his experiences.

Northern Ireland peace broker George Mitchell last night went into the latest round of his marathon review of Ulster's Good Friday agreement by meeting Irish premier Bertie Ahern in Dublin.

The former United States senator briefed Mr Ahern on his nine weeks of talks so far with Northern Ireland's political party bosses ahead of making similar reports to Mr Tony Blair and President Clinton.

The senator meets the Prime Minister in London today and Mr Clinton in Washington later this week.